As is well known, polyacetal resins are used in a number of end-use applications as engineering plastics due to their excellent physical properties, such as their excellent mechanical and electrical properties, as well as their excellent chemical resistance and heat resistance properties. However, polyacetal resins having further property improvements are needed as their use expands into new applications.
For example, further improvements are, in some cases, required in the weather resistance properties of polyacetal compositions. In particular, the properties of exterior parts of automobiles and parts of electrical appliances and business machines are likely to deteriorate when subjected to ambient conditions of use, such as solar rays, rain, fluorescent lamps and air. Thus, parts formed of polyacetal resins may, during use, become discolored, lose their surface smoothness resulting in a reduction of gloss, and/or experience crack formation on the parts' surfaces. Such deterioration may thus impair the parts' appearance and/or mechanical properties.
It has been proposed previously to improve the weather (UV-light) resistance of polyacetal resins by incorporating stabilization "packages" of various weather (UV-light) stabilizers therein. Although some effects are obtained by use of conventional weather stabilizer packages, the surface conditions (e.g. cracks, glass and the like) and the mechanical properties of polyacetal moldings after being exposed to solar rays (ultraviolet rays), rain, and the like for prolonged time periods are not always satisfactory. Moreover, increased weather resistance properties have more recently been required to protect the polyacetal parts against severe environmental pollution problems, such as acid rain.
Simply increasing the amount of weather stabilizers only results in limited improvements to the weather resistance properties of polyacetal resin compositions. In addition, an increase in the amount of weather stabilizers usually impairs the mechanical properties (such as tensile strength and bending strength) of the polyacetal composition. The stabilizers furthermore may adhere to the mold to form mold deposits during molding thereby reducing the dimensional accuracy and/or detracting from the appearance of the moldings. As a result, an increase in the time needed to clean the mold ensues which, in turn, reduces the efficiency of the molding operation. Another problem that is encountered is that weather stabilizers may "bleed" onto the surfaces of the moldings which detrimentally affects their appearance. Thus, the conventional weather stabilization techniques are still unsatisfactory in many respects.
The present invention is directed towards improving the weather (UV-light) stability of polyacetal resin compositions (particularly for molded parts used outdoors).
Broadly, the present invention is directed to normally solid weather-resistant polyacetal compositions, and molded parts formed thereof, comprised of a polyacetal base resin, and a weather-resistant effective amount of a weather stabilization package which includes a UV-light stabilizer and a basic compound having at least one tertiary nitrogen atom and at least one metal carboxylate group in its molecule.
More specifically, the present invention relates to weather-resistant polyacetal resin compositions and molded parts comprised of:
(A) a polyacetal base resin;
(B) between 0.01 to 5% by weight, based on the entire weight of the composition, of a weather (UV-light) stabilizer, and
(C) between 0.01 to 3% by weight, based on the entire weight of the composition, of a basic compound containing at least one tertiary nitrogen atom and at least one metal carboxylate group in the molecule.
Optionally, between 0.1 to 5% by weight, based on the entire weight of the composition, of a colorant may be added to the above-described components.